Some of the sensory pathways convey information about only a single type of sensory information. Thus, one pathway is influenced only by information from mechanoreceptors, whereas another is influenced only by information from thermoreceptors. The ascending pathways in the spinal cord and brain that carry information about single types of stimuli are known as the specific ascending pathways. The specific pathways pass to the brainstem and thalamus, and the final neurons in the pathways go from there to different areas of the cerebral cortex (Figure 9-6). (The olfactory pathways are an exception because they go to parts of the limbic system rather than to the thalamus and because they terminate in the limbic system.) By and large, the specific pathways cross to the side of the central nervous system that is opposite to the location of their sensory receptors so that information from receptors on the right side of the body is transmitted to the left cerebral hemisphere and vice versa.

The specific ascending pathways that transmit information from somatic receptors—that is, the receptors in the framework or outer walls of the body, including skin, skeletal muscle, tendons, and joints— go to the somatosensory cortex, a strip of cortex that lies in the parietal lobe of the brain just behind the junction of the parietal and frontal lobes (Figure 9-6).

The specific pathways from the eyes go to a different primary cortical receiving area, the visual cortex, which is in the occipital lobe, and the specific pathways from the ears go to the auditory cortex, which is in the temporal lobe (Figure 9-6). Specific pathways from the taste buds pass to a cortical area adjacent to the face region of the somatosensory cortex. As we have indicated, the pathways serving olfaction have no representation in the cerebral cortex.

Finally, the processing of afferent information does not end in the primary cortical receiving areas but continues from these areas to association areas of the cerebral cortex.

In contrast to the specific ascending pathways, neurons in the nonspecific ascending pathways are activated by sensory units of several different types (Figure 9-7) and therefore signal general information. In other words, they indicate that something is happening, without specifying just what or where. A given second-order neuron in a nonspecific pathway may respond, for example, to input from several afferent neurons, each activated by a different stimulus, such as maintained skin pressure, heating, and cooling. Such pathway neurons are called polymodal neurons. The nonspecific pathways, as well as collaterals from the specific pathways, end in the brainstem reticular formation and regions of the thalamus and cerebral cortex that are not highly discriminative, but are important in the control of alertness and arousal.

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.